In the petrochemical industry there is a growing demand for normally gaseous olefins, such as ethylene and propylene. In order to cope with this increasing demand, plants for the manufacture of the lower olefins have been enlarged both in number and size. Generally, most such plants subject a hydrocarbon feedstock, e.g. ethane, C.sub.3 -C.sub.5 paraffins, naphtha or gas oil, to a thermal cracking treatment in the presence of steam. In the present description and claims, this thermal cracking treatment in the presence of steam will further be referred to as "steam-cracking"
The increase in consumption of gas oil and lower boiling fractions caused by increased olefin production may result, and in some instances has already resulted, in a shortage of suitable feedstocks for steam-cracking. Moreover, an increase in consumption of the relatively lighter tractions of the crude oil is coupled with an increased amount of heavier fractions becoming available which, unless disposed of as fuel, has to be converted in some way or another into more valuable products. Consequently, it would be advantageous if the relatively heavier fractions could be processed economically in such a manner that additional quantities of the required lower olefins are produced. In this fashion, shortages of gas oil, naphtha, and other light fractions could be minimized, while better use would be made of the heavy fractions.
In one proposed process, olefins are produced by hydrogenating a petroleum distillate containing aromatic hydrocarbons, e.g. a vacuum distillate boiling in the range of 300.degree.-650.degree. C, in the presence of a hydrogenation catalyst to at least partially saturate the aromatic hydrocarbons, and then steam-cracking the resulting hydrogenated product (cf. L.K. patent specification No. 1,361,671).
However, use of these feedstocks produces a bituminous residue fraction (vacuum residue) with a relatively high viscosity, which is difficult to dispose of. Additionally, in order to achieve the required saturation of the aromatic hydrocarbons, the catalytic hydrogenation of these heavier distillates has to be performed at high pressures and/or temperatures requiring the use of special hydrotreatment equipment.
Finally, the subsequent steam-cracking of the hydrotreated vacuum distillates generally results in the formation of considerable amounts of tar, with attendant fouling in furnaces and downstream equipment. Although it may be possible to obtain acceptable run lengths between subsequent decokings, in spite of the increased tendency to foul, by suitable design of furnaces and downstream equipment, significant costs would be incurred thereby at the expense of the overall economy of the process.
It has now been found that a more economic procedure can be followed, according to which a substantial part of the barrel of crude oil is converted into valuable products, while the aforesaid problems are minimized.